LeetCode 48: Rotate Image

You are given an n x n 2D matrix representing an image.
Rotate the image by 90 degrees (clockwise).
Follow up:
Could you do this in-place?

public class Solution {
    public void rotate(int[][] matrix) {
        int n = matrix.length;
        
        if (n <= 1)
            return;
        
        // One by one swap clockwise
        // Java can't pass by pointer.
        for (int i = 0; i < n/2; i++)
        {
            for (int j = i; j < n-1-i; j++)
            {
                int tmp = matrix[i][j];
                matrix[i][j] = matrix[n-j-1][i];
                matrix[n-j-1][i] = matrix[n-i-1][n-j-1];
                matrix[n-i-1][n-j-1] = matrix[j][n-i-1];
                matrix[j][n-i-1] = tmp;
            }
        }
    }
}